Have you requested an early review of the EHCP?
Have you looked at other SS who admit from 5 including independent ones and schools out of area?
Supervision, including e.g. 1:1, can be provided in ARPs. Many LAs and schools will tell you it can’t be but it can.
If you want DS to attend school full time, he can unless the school is formally suspending DS each time. If the school is not formally suspending DS, what they are doing is unlawfully, informally excluding him. You don’t have to allow this to continue. A formal suspension instead of an unlawful, informal exclusion will a) provide evidence of unmet needs to help you pursue additional support &/or an alternative placement, b) force the school to follow due process, c) limit the number of days the school can suspend for, d) allow you to challenge any suspension, and e) ensure DS receives alternative education for longer suspensions.
If DS can’t attend school full time and turned last term, the LA has a duty to ensure he still receives a full-time education. On their website, IPSEA has a model letter you can use to request alternative provision. And if DS can’t attend school, the LA is still responsible for the provision in the EHCP.
Secondly, if the provision detailed, specified and quantified in F of the EHCP isn’t being provided, have you contacted the LA? It is the LA who is ultimately responsible for ensuring the provision is provided. IPSEA also has a model letter you can use for this. If that doesn’t work, you need a pre-action letter. SOSSEN can help with this free of charge. Although there is a wait, so you may want to look elsewhere. Then, if that fails, judicial review proceedings will resolve the situation.
However, enforcement action is only possible if the provision is detailed, specified and quantified. If it isn’t, you should request an early review of the EHCP in order to tighten it up.